Mitochondrial integrity and antioxidative enzyme activity are two of the determinants of intracellular reactive oxygen species (ROS) accumulation probably underlying the aging mechanism. In this study, epigallocatechin-3-gallate (EGCG) was examined for its antiaging effect on human diploid fibroblasts (HDF). EGCG was evaluated for its cytotoxicity, and LC50 values were 78.0 and 84.4 microM for young and old HDF, respectively. HDF treated with EGCG at 25 and 50 microM for 24 h considerably increased catalase, superoxide dismutase (SOD)1, SOD2, and glutathione peroxidase gene expressions and their enzyme activities, thus protecting HDF against H2O2-induced oxidative damage, accompanied with decreased intracellular ROS accumulation and well-maintained mitochondrial potential. Moreover, HDF treated with EGCG at 12.5 microM for long term showed less intracellular ROS with higher mitochondrial potential, more intact mitochondrial DNA, much elevated antioxidative enzyme efficiency, and more juvenile cell status compared to those of the untreated group. Taken together, in this study we investigated the effects of EGCG in the regulation of mitochondrial integrity and antioxidative enzyme activity of HDF, suggesting that EGCG can be considered one of the possible antiaging reagents in the future.